At the invitation of Dr. Jan-Leen Kloosterman of the Delft University of Technology (Technische Universiteit Delft, TU Delft) I attended a symposium on thorium in molten-salt reactors held on April 17, 2015.
Other participants included David LeBlanc of Terrestrial Energy and Leslie Dewan of Transatomic Power.
My objective was to make the case that the thorium fuel cycle, integrated with the molten-salt reactor, represented the best path forward for humanity’s energy future. I began with a review of the dates of Glenn Seaborg’s pioneering work on thorium as well as the early technological achievements at Oak Ridge National Laboratory. I stated my opinion that political mistakes and poor information exchange led to the cancellation of molten-salt reactor work, but that key questions about materials compatibility were answered satisfactorily during that time to have allowed work to go forward. Materials were not the reason that work was cancelled.
I also asserted that nuclear safety has only grown in importance since the 1970s and that emphasis on safety will continue to favor the development of molten-salt reactors; that they offer the best confluence of safety, performance, and technological readiness of any known reactor technology.
I then went through the now-standard introduction of thorium, its composition and abundance, and the manner in which it is utilized for fission by absorption of a neutron to form uranium-233. I described how both uranium-238 and thorium-232 could absorb neutrons to form fissile nuclides, but that only uranium-233 from thorium produced sufficient neutrons in thermal fission to continue the conversion reactions.
But then I described things from a different perspective. Assuming that molten-salt reactor technology was used for any of the three feasible approaches to fuel utilization, I described their relative merits versus one another.
If a molten-salt reactor was built that was meant to be fueled by low-enrichment uranium, such as the IMSR being designed by Terrestrial Energy, then a world powered by such a reactor will require much more uranium mining and enrichment capabilities. It will bear a strong resemblance to the uranium use of today’s reactors, and the issue of the spread of uranium enrichment technology will continue to be a concern.
Another option would be to have a fast-spectrum molten-salt reactor, perhaps using a chloride salt, that was meant to implement the uranium-plutonium fuel cycle. Such a reactor would not require uranium mining due to the large inventories of depleted uranium remaining from many decades of uranium enrichment. It would have the potential for a significant breeding ratio, but it would have the largest fissile inventory of any of the fuel cycle options. This could be a concern.
The best approach, I put forth, was to utilize the thorium fuel cycle in a thermal-spectrum MSR like the LFTR design. It would also require no uranium or thorium mining, due to the recovery of thorium from rare-earth mining. It would have the minimum inventory of fissile material and that fissile material would all consist of uranium-233.
I advised the assembled group that we should “choose our fissile currency of the future wisely”, since both uranium enrichment and plutonium generation and extraction are paths that have been weaponized. Uranium-233 was not weaponized because of the unavoidable contamination of the fissile material by uranium-232, and this would render the material unattractive to a nation-state, which would choose the simpler path of uranium or plutonium, or a subnational group, which could never undertake a program of testing and development.
Plutonium or thorium? Plutonium could never be isotopically diluted like uranium-233 could in the event of an emergency, and only the thorium-uranium-233 cycle avoided the production of plutonium in the first place. I asserted that uranium-233 represented the safer approach since uranium-233 was never weaponized like plutonium was.
I then described the LFTR technology, the conceptual design, and the technology challenges that lay ahead. I expressed my confidence that we will be able to find acceptable solutions of the residual challenges of MSR technology and that we will be able to realize its tremendous advantages.
Additional notes on the other talks by Quincy Sorensen are available here:
There is a growing and appreciable base of interest in thorium MSR in the Netherlands, and it is underpinned by the high quality research at TU Delft as well as the molten-salt irradiation experiments that will by done by NRG at their Petten facility in this calendar year involving LiF-BeF2-ThF4 samples in the HFR facility. The symposium was arranged in large part by Theo Wolters, a successful businessman in the Delft area with ties to the university and to Dr. Kloosterman.
The conference was quite well-attended, far more than any other thorium-related conference I have previously attended. I would estimate that there were about 200 people in the audience of the large auditorium where it was held. There were several members of the Dutch parliament there, which was very gratifying.
The night after the conference we met for dinner as a group and Theo was in very good spirits. He encouraged everyone to cooperate and not to work against each other.
The Saturday after the conference my wife and I spent sightseeing in Amsterdam, and Sunday she flew back to the US.
On Monday, I took the train north from Delft to the Petten facility, which is where NRG operates their HFR facility. I met with Sander de Groot and Lucas Pool while I was there. First we had lunch and then Sander took us on a walking tour of the facility. He showed the HFR and the hot-cell facilities across the street where targets are dissolved for molybdenum extraction. He showed the Mallickrodt buildings, which is the company there and also in the US that process molybdenum into generators for sale to hospitals. The site reminded me a lot of the Sellafield facility, on a much smaller scale. But both were on the sea coast, rather far away from populated areas, and appeared to have plenty of room for expansion if that was desired.
We returned to a conference room and continued our discussions about collaboration. Sander did a presentation where he showed how they would soon be loading molten-salt (LiF-BeF2-ThF4) samples into the HFR for an extended irradiation campaign. He lamented the fact that these samples would be in a lower-flux region of the HFR and that it would take a long time for the thorium to transmute to U-233 and to fission. I noted that the lower flux would allow most of the protactinium-233 to decay to U-233 rather than to absorb a neutron before the decay was complete, and that that fact would make it a more accurate simulation of a real thorium MSR. Sander also said that the effort to get these molten-salt samples into the HFR was initiated almost eight years ago, and that it was funded out of government R&D funds, and so it ended up having a lower priority than he would have liked.
In his description of the HFR, I noticed several similarities and differences with the OPAL reactor at ANSTO. Both reactors use square fuel assemblies and have a square core design. Both are cooled and moderated by light water. But there is no heavy-water reflector around the HFR like there is in OPAL. Rather, the irradiation locations are located inside the fuel assembly array, while all of OPAL’s irradiation locations are outside of the square core. This might mean that higher fluxes would be achieved in HFR, in addition to its higher thermal power of 45 megawatts. But the most interesting thing that he said was about gas, namely that it was possible to inject gas into a sample and to withdraw that gas during testing. Indeed, this is exactly what they planned to do as they evaluated the behavior of some Chinese TRISO-coated particle fuel that was intended for a gas-cooled reactor. They would inject gas and withdraw it and examine what they saw from the gas, and from that data they would assess the integrity of the Chinese fuel.
I left Petten and travelled to Delft. That night I met for dinner with Theo and Hans again and the next day returned to the US, very grateful for the wonderful time I spent in the Netherlands and the new friends I made there.